The present invention relates in general to a circuit chip package which employs low melting point solder to transfer heat away from circuit chips, and a method for assembling the same.
Multi-chip Units, otherwise known as MCUs, are circuit modules using interconnect mediums, such as metal/ceramic, copper/polymide or copper/FR4, which house several microchips and are used extensively in computers. The MCU protects the chips from the environment and heat, and also supports the chips' needs for power and signal interconnections between other chips within the MCU or other MCUs elsewhere in the computer.
Numerous techniques have been employed in the past to direct heat away from the chips in an MCU so that they are maintained within a permissible range of operating temperatures. One prior technique known as bottom side cooling employs heat removal structures which contact the bottom sides of the chips and remove heat through the MCU base. A drawback to this technique is that the heat removal structures take up space in the MCU base which interferes with the signal and power interconnects and prevents high density chip packing configurations.
An alternative to bottom side cooling which avoids this drawback is called top side cooling in which heat is removed from the sides of the chips opposite to their bottom interconnect sides. Top side cooling techniques typically employ fluid cooling wherein a gas or liquid is caused to flow over the exterior portions of the chip to draw away heat. In MCUs, a plurality of circuit chips are packaged within a protective housing and direct fluid cooling of the chips is difficult since cooling fluid cannot be passed through the MCU housing without specific structural modifications. A known solution to this problem is to couple each of the circuit chips thermally to the MCU housing so that the heat generated by the chips is transferred to the housing which can be cooled using any suitable technique. However, it is difficult to connect each of the circuit chips rigidly to the MCU housing because some way must be provided to accommodate minute dimensional differentials between each of the like circuit chips and the housing. It is also highly desirable to accommodate the more significant dimensional differentials between dissimilar chips and microassemblies. To remedy this, spring elements have been employed which attempt to provide good thermal contact between each of the circuit chips and the MCU housing while accommodating for chip to housing dimensional variances. However, since the spring elements are inherently flexible, they may be subjected to motion which could impair thermal contact quality. Also this technique becomes costly and laborsome for situations having significant height differentials between chips or microassemblies due to the different springs or parts required.
In view of the foregoing, what is needed is a technique for thermally connecting circuit chips to a MCU housing which provides a good thermal path between the chips and the housing that accommodates for chip to housing dimensional variances without requiring complicated arrangements of mechanical parts that would impair the assembly process, and without interfering with the power and circuit interconnections.